One well-known method for the exposure of photographic photosensitive elements is an image forming method of the scanner system comprising the steps of scanning an original to produce image signals, subjecting a photographic silver halide photosensitive element to exposure in accordance with the image signals, and forming a negative or positive image corresponding to the image of the original.
There is a desire to have a procedure of providing outputs of a scanner to a film and directly printing on a printing plate without a transfer step as well as for a scanner photosensitive element having ultrahigh contrast with respect to a scanner light source having a soft beam profile.
There are known a number of photosensitive elements having a photosensitive layer on a support wherein images are formed by imagewise exposure. Among these, a technique of forming images through heat development is known as a system capable of simplifying image forming means and contributing to environmental protection.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the photomechanical process field to reduce the quantity of spent solution. Needed in this regard is a technology relating to photothermographic elements for use in reprophotography which can be effectively exposed by means of laser scanners or laser image setters and produce distinct black images having high resolution and sharpness. These photothermographic elements offer to the customer a simple thermographic system which eliminates a need for environmentally detrimental solution type chemical agents.
The technology of forming images through heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969. These photothermographic elements generally contain a reducible non-photosensitive silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), and a reducing agent for silver, typically dispersed in an organic binder matrix. Photothermographic elements are stable at room temperature.
When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the reducible silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
Photothermographic elements of this type are well known in the art. In most of these elements, photosensitive layers are formed by applying coating solutions based on organic solvents such as toluene, methyl ethyl ketone (MEK) and methanol, followed by drying. The use of organic solvents is not only harmful to workers in the manufacturing procedure, but disadvantageous because of the cost for recovery and disposal of the solvents.
It is contemplated to form photosensitive layers using coating solutions based on water solvent which eliminates such concern. Such photosensitive layers are sometimes referred to as "aqueous photosensitive layers," hereinafter. For example, JP-A 52626/1974 and 116144/1978 disclose the use of gelatin as the binder. JP-A 151138/1975 discloses polyvinyl alcohol as the binder. Further, JP-A 61747/1985 discloses a combined use of gelatin and polyvinyl alcohol. Besides, JP-A 28737/1983 discloses a photosensitive layer containing water-soluble polyvinyl acetal as the binder.
It is true that the use of these binders has great environmental and economical advantages in that photosensitive layers can be formed using coating solutions based on water solvent.
However, the use of such polymers as gelatin, polyvinyl alcohol and water-soluble polyacetal as the binder has the following drawback. Dehydration shrinkage and thermal expansion of the binder simultaneously occur upon heat development, which behavior is different from the thermal expansion behavior of the support, causing films to be wrinkled. The wrinkled films are inadequate for color printing where the films are used in register with each other.
There is a desire to have a photothermographic element which is an aqueous photosensitive element advantageous from the environment and economical aspects and has good coating quality and improved dimensional stability enough to avoid wrinkling upon development.